What happened to our Lauryn?

My boys and I are heartbroken. Our 1 and 1/2 year old NH Red hen seemed slow moving on Tuesday, we brought her inside, cleaned her vent area and noticed she seemed "Full". We thought she had to lay an egg. Today she free ranged the property and when we checked her tonight had a larger fuller feel to her. We looked up online what might be going on with her and thought her penguin like walk, slowness and lethargy and full abdomen might indicate an egg bound situation and gave her an internal exam after a warm soak in the sink. She pooped quite a bit and then died in my hands within 5 minutes or so of this vent check. We don't understand what happened to our Lauryn. Our other 5 chickens (various breeds) seem fine. What could have happened?

Welcome to BYC. More than likely she had some sort of reproductive issue such as cancer, internal laying, or egg yolk peritonitis that may have caused fluid from the liver to gather in her abdomen. It is called ascites and it can cause swelling of the lower abdomen and shortness of breath. There was probably nothing you could do for her. Her penguin walk and lethargy could also be a sign of internal laying/peritonitis, as well as being egg bound. Sorry for your loss. If you want to get a cause of death, you can refrigerate her body and send it to your state vet for a necropsy. Sometimes it is hard to do that with a loved pet, though. Here is a link for that: